Når man trækker vejret dybt, bliver kroppen mere rolig.

Når means when in the sense of something general, repeated, or normally true.

In this sentence, it means: whenever this happens, this is the result.

• Når = when
• Hvis = if

So:

• Når man trækker vejret dybt... = When you breathe deeply...
• Hvis man trækker vejret dybt... = If you breathe deeply...

Hvis would sound more conditional. Når sounds more like a general fact.
You might also see da, but da is usually used for a specific time in the past, not for a general truth like this.

Here, man means one, you, or people in general.

It does not mean a man in this sentence.

Danish uses man very often for general statements:

• Man lærer hurtigt. = You / One learn(s) quickly.
• Når man trækker vejret dybt... = When you / people breathe deeply...

In everyday English, this is often best translated as you, even though Danish uses man.

Because at trække vejret is the normal everyday Danish expression for to breathe.

There is also the verb at ånde, but that is less common in ordinary everyday speech and can sound more formal, literary, or technical.

So for a learner, it is best to remember:

• at trække vejret = to breathe

Treat it as a fixed expression.

In at trække vejret, the word vejret is simply part of the fixed expression.

Yes, -et is the definite ending, but in practice you should learn the whole phrase as one unit:

• trække vejret = breathe

Even though vejr usually means weather, this expression does not mean you should translate it word for word. It is an idiomatic phrase.

Because dybt is describing how someone breathes, so it functions like an adverb: deeply.

Danish often uses the -t form of an adjective when it is used adverbially.

Compare:

• en dyb sø = a deep lake
• at trække vejret dybt = to breathe deeply

So:

• dyb = deep
• dybt = deeply

Because Når man trækker vejret dybt is a subordinate clause, and bliver kroppen mere rolig is the main clause.

Danish normally separates those parts with a comma in writing.

So the structure is:

• subordinate clause: Når man trækker vejret dybt
• main clause: bliver kroppen mere rolig

This is because of normal Danish V2 word order in main clauses.

When the sentence begins with something other than the subject — here, the whole clause Når man trækker vejret dybt — the finite verb of the main clause comes next.

So you get:

• Når man trækker vejret dybt, bliver kroppen mere rolig.

If you put the subject first, then you would say:

• Kroppen bliver mere rolig, når man trækker vejret dybt.

Both are correct. The word order changes because Danish likes the finite verb in second position in the main clause.

Because bliver means becomes / gets, so it shows a change.

The sentence is saying that the body changes into a calmer state.

Compare:

• Kroppen bliver rolig. = The body becomes calm.
• Kroppen er rolig. = The body is calm.

So bliver is the right choice when the meaning is becomes calmer.

Because kroppen is the definite form:

• en krop = a body
• kroppen = the body

In Danish, the definite article is usually added as an ending to the noun.

Also, Danish often uses the definite form in places where English might use a possessive or a more general expression. So kroppen is very natural here.

Because rolig agrees with kroppen, which is a singular common-gender noun.

After verbs like være and blive, adjectives still agree with the subject:

• kroppen er rolig
• huset er roligt
• børnene er rolige

So in this sentence:

• kroppen = singular, common gender
• therefore: rolig

Yes.

Danish can make comparatives in two common ways:

• roligere = calmer
• mere rolig = more calm

For rolig, many speakers would probably find roligere more natural in everyday Danish.
So:

• bliver kroppen roligere often sounds more idiomatic

But mere rolig is still understandable, and learners may meet both patterns in Danish.